The Vatican, Rome (CNN) — On Sunday, for the first time in history, the Catholic Church will canonize two popes on the same day.
Pope Francis will preside over a special ceremony that is expected to draw upwards of a million pilgrims, who will gather in St. Peter's Square to witness Pope John Paul II and Pope John XXIII enter the celestial community of Catholic saints.
Here's a bit about the Catholic Church's canonization process.
What is a saint, and how many are there?
Catholics believe a saint is someone who lived a holy life and who’s already in heaven. Saints are considered role models for people still on Earth, and are capable of interceding with God on someone’s behalf when a request for help is made in prayer.
The actual number of saints is impossible to calculate. One well-known work called "Lives of the Saints" lists 2,565 Catholic saints, but that doesn’t count thousands of others celebrated in local regions all over the world.
The Catholic Church has a feast, All Saints’ Day, on November 1 to honor the countless saints who aren’t formally canonized.
So how does one become a saint?
In one sense it’s a democratic process, beginning with a grassroots conviction that a given person lived a holy life. From there, things unfold in three stages. First, Church officials make a study of the person’s life.
Next, one miracle after the candidate’s death is required for be beatification -- and another for canonization. Usually the miracles are healings, which must be instantaneous, permanent, and complete, in addition to scientifically inexplicable. Catholics see the miracle as God’s seal of approval, a way of verifying that the saint really is in heaven.
Pope John XXIII was known as a holy -- and very funny -- man. When a reporter once asked how many people work at the Vatican, he answered, "About half."
John has been credited with one miracle, the healing of an Italian nun in 1966, but Pope Francis waived the requirement for a second miracle -- as is the papal prerogative.